Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/20—Automatic control or regulation of cells

Definitions

• the present invention relates to a process for the precise regulation of a low alumina content in an igneous electrolysis tank for the production of aluminum according to the Hall-Héroult process, this regulation also having the aim of maintaining the Faraday yield at a level high, at least equal to 94%.
• This parameter is generally the variation of the internal resistance, or, more exactly, of the internal pseudo-resistance which is equal to:
• Eo being an image of the counter-electromotive force of the tank, the value of which is generally assumed to be 1.65 volts, U the voltage across the terminals of the tank and J the intensity passing through it.
• the alumina concentration is fixed in the range of 2 to 8%.
• the variation ⁇ V, as a function of time t, of the voltage at the terminals of each tank is measured, it is compared with a predetermined value and the rate of supply of alumina is modified to bring the ⁇ V / t to the value of instructions.
• the disadvantage of this process is that its sensitivity varies with the alumina content, which is precisely minimal in the interval used, from 3 to 5% of Al 2 O 3 (table p. 8).
• the alumina content is also fixed in the range of 2 to 8% and, preferably, 4 to 6%.
• the tank is supplied for a predetermined time tl with an amount of alumina greater than its theoretical consumption, until a predetermined alumina concentration is obtained (for example up to 7%), then the supply is switched at a rate equal to the theoretical consumption for a predetermined time t 2 , then the feeding is stopped until the appearance of the first symptoms of anode effect ("packaging"), and the feeding cycle is resumed higher than theoretical consumption.
• the alumina concentration varies, during the cycle, from 4.9 to 8% (example 1) or from 4.0 to 7% (example 2).
• slope calculation is based on successive measurements of the internal resistance R i , at equal time intervals, on the evaluation of the slope dRi / dt of variation of R. as a function of time, and the comparison of R. on the one hand and of dRi / dt on the other hand, to set values, and on the modification of the rate of introduction of alumina, so as to reduce dRi / dt and R. to the set values.
• the search for the optimal operating mode that is to say the search for the operating parameters of the electrolytic cells giving the best cost price, or the most important profit margin for a given investment, has always been a permanent concern. for those skilled in the art.
• the object of the invention is an improvement of the process for the precise regulation of a low alumina content in the electrolysis tank, making it possible to significantly improve the Faraday yield.
• object of our aforementioned patent on our modern electrolysis cells operating at 175,000 or 280,000 amperes, with a so-called "acid" bath composition, this is that is to say a bath comprising more than 8% by weight of aluminum fluoride AlF 3 in excess relative to the neutral cryolite of formula Na 3 AlF 6 , we have found, contrary to the general opinion of the specialists that we indicated previously, that, despite the increase in temperature of the electrolysis bath, the current yield increased rapidly when the alumina content of the bath decreased.
• the energy consumption per tonne of aluminum produced can be put according to the Faraday yield, F, and the voltage at the terminals of a tank, U, in the form:
• this optimal alumina content is very close to the minimum content below which appears the "anode effect", also called “packaging” or “polarization” , which results in a very sharp rise in the voltage across the cell and in the temperature of the electrolysis bath, and in the release in significant quantities of fluorinated products from the decomposition of the electrolysis bath.
• This parameter P is evaluated from the internal pseudo-resistance of the tank, R., itself defined by: where U is the voltage across the tank (in volts)
• Eo is a standard value, in volts, of the dynamic counter-electromotive force of the tank, generally between 1.5 and 2.0 volts, and most often of the order of 1.65 to 1.75 volts
• D is the alumina content drift of the electrolysis bath, expressed for example in weight percent per hour
• P is expressed by the formula: (P being expressed in micro-ohms per second and per% weight per hour).
• the regulation of the tank according to the invention consists in staying as long as possible in an alumina content zone, not necessarily known in a precise manner, but such that P is as close as possible to a Po value that one s 'is fixed beforehand.
• This initial period which generally lasts only a few minutes, corresponds to the end of dissolution of the excess alumina introduced during the overeating period and not immediately assimilated by the bath.
• Another method is to add a period of a few minutes at nominal rate after overeating before going on to undereating. After this initial period, the alumina content of the bath decreases all the more quickly the slower the feed rate, and, in parallel, the measured dR./dt slope increases.
• the alumina content drift, D, counted in% weight per hour, is then proportional to: where C - is the rate of undernourishment counted in kg of Al 2 O 3 introduced per second and C N is the nominal rate of feed (counted in the same units). Any other coherent system of units can of course be used, for example the inverse of the time separating the introduction of 2 consecutive doses of alumina.
• Q Al 2 O 3
• Q (liquid bath) is the weight of liquid bath, capable of dissolving alumina, contained in the crucible of the tank (for information, if the weight of liquid bath is measured in kg, around 30 J where J is the electrolysis intensity counted in kA): note that the time constant for the melting or solidification of the bath at the slope is very large (generally of the order of several hours), this quantity does not vary only very slowly over time.
• the target alumina content being close to the limit content triggering the appearance of a polarization of the tank, it is essential that after operating at nominal rate, the readjustment is done by preceding the search phase of the operating point (characterized by Po), during an under-timing, by an over-timing period which makes it possible to move away from this limit content before triggering the search.
• Po search phase of the operating point
• the regulation method according to the invention can be used only during part of the operating time of the tank, and preferably when the tank is stable.
• K 1 is an "economic” coefficient synthesizing the economic conditions of the moment (in particular cost of energy compared to other cost items , excluding alumina)
• K 2 is a "technical” coefficient synthesizing the technological and physicochemical characteristics of the tank (K 2 is substantially independent of K 1 )
• this parameter Po will be maintained between the limit values of 2,100 J and 100 J.
• K 1 is approximately equal to: (an estimate of K 1 at ⁇ 20% is more than enough to get close enough to the economic optimum).
• this factor dF / d (Al 2 O 3 ) must be determined experimentally for each type of tank and for the various types of bath used (low acid baths, with less than 8% excess of AlF 3 , or very acid baths, with more than 8% excess of AIF 3 or with sub-additives such as LiF and MgF 2 ). Once determined, it no longer depends, as a first approximation, on economic conditions.
• the invention has been applied to a series of electrolytic cells operating at an intensity of 280 KA, at a voltage of 4.10 volts per cell and giving a Faraday yield of 95.0% for an average alumina content in the bath. electrolysis equal to 2.3%, previously regulated according to the method of our patent FR 2 487 386 already cited (process called "slope calculation").
• the gain on the cost price was 20 F per tonne of aluminum produced.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Cell Separators (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Weting (AREA)
• Table Devices Or Equipment (AREA)
• Manufacture And Refinement Of Metals (AREA)

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Citations (4)

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• 1985
  • 1985-05-07 FR FR8507319A patent/FR2581660B1/fr not_active Expired
• 1986
  • 1986-04-24 IN IN321/CAL/86A patent/IN164906B/en unknown
  • 1986-04-29 GR GR861139A patent/GR861139B/el unknown
  • 1986-05-02 IS IS3095A patent/IS1347B6/is unknown
  • 1986-05-05 EP EP86420118A patent/EP0201438B1/de not_active Expired
  • 1986-05-05 US US06/859,907 patent/US4654129A/en not_active Expired - Fee Related
  • 1986-05-05 AT AT86420118T patent/ATE44165T1/de not_active IP Right Cessation
  • 1986-05-05 DE DE8686420118T patent/DE3664058D1/de not_active Expired
  • 1986-05-05 NZ NZ216051A patent/NZ216051A/xx unknown
  • 1986-05-06 CN CN86103165A patent/CN1006307B/zh not_active Expired
  • 1986-05-06 AU AU57157/86A patent/AU576152B2/en not_active Ceased
  • 1986-05-06 ZA ZA863380A patent/ZA863380B/xx unknown
  • 1986-05-06 NO NO861806A patent/NO172192C/no unknown
  • 1986-05-06 PL PL1986259354A patent/PL144950B1/pl unknown
  • 1986-05-06 ES ES554683A patent/ES8800733A1/es not_active Expired
  • 1986-05-06 BR BR8602039A patent/BR8602039A/pt not_active IP Right Cessation
  • 1986-05-06 CA CA000508520A patent/CA1251417A/fr not_active Expired
  • 1986-05-07 TR TR25030/86A patent/TR22683A/xx unknown
  • 1986-05-07 HU HU861893A patent/HU205632B/hu not_active IP Right Cessation
  • 1986-05-07 JP JP61104591A patent/JPS61261490A/ja active Pending
  • 1986-05-07 OA OA58855A patent/OA08324A/xx unknown
• 1987
  • 1987-04-09 MY MYPI87000457A patent/MY101644A/en unknown

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